Kinetic Modeling of Homogeneous Mercury Oxidation:  The Importance of NO and H₂O in Predicting Oxidation in Coal-Derived Systems

Abstract

This paper develops and evaluates an elementary reaction mechanism for homogeneous Hg0 oxidation that accounts for major interactions among Cl-species and other pollutants in coal-derived exhausts. Most importantly, interactions among NO and Cl-species were found to exert a strong and previously unrecognized impact on homogeneous Hg0 oxidation under some but not all conditions. The proposed oxidation mechanism is subjected to quantitative evaluations against all the available laboratory datasets that characterize homogeneous Hg0 oxidation when HCl is the primary chlorinated species. The simulations depict the reported extents of oxidation for broad ranges of HCl and temperature within useful quantitative tolerances without any heuristic parameter adjustments. The predicted pool of Cl-atoms was found to be governed by the chemistries of moist CO oxidation, Cl-species transformations, and NO production. However, an artificial initiation scheme was needed to depict the temperature dependence observed in one set of literature data. This indicates that either the kinetic mechanisms are incomplete or that heterogeneous initiation came into play under these test conditions. The evaluations further show that Hg oxidation is primarily through a Cl atom recycle process, with Cl and Cl2 concentrations both playing an important role. Oxygen weakly promotes homogeneous Hg oxidation, whereas moisture is a stronger inhibitor. NO can promote or inhibit homogeneous Hg oxidation, depending on its concentration. In the presence of NO, extents of Hg oxidation increased for progressively faster quenching. Conversely, without NO, extents of oxidation diminished for faster quenching.

Mechanisms and Models Describing Sodium and Lead Scavenging by a Kaolinite Aerosol at High Temperatures

“…Chlorine, for example, has a complicated speciation path (Gullet et al 2000), which in turn has a significant impact on mercury speciation (Niksa and Helble 2001;Senior et al 2002). However, the major impact of SO 2 and chlorine observed on semivolatile metal (i.e., Na and Pb) capture by kaolinite (in the temperature range investigated in this work) has been to change the dew point of the reactive metal species, and thus change the time available for reaction (Mwabe and Wendt 1996;Scotto et al 1992;Davis and Wendt 2000a;Linak et al 1995).…”

Mercury Control with Calcium-Based Sorbents and Oxidizing Agents

“…The predominant forms of mercury in coal-fired flue gas are elemental (Hg o ) and oxidized (HgCl 2 ) [1][2][3]. The percentage of oxidized mercury in the stack effluent of a particular power plant depends on the coal type, combustion efficiency, and the pollution control equipment used.…”

“…The percentage of oxidized mercury in the stack effluent of a particular power plant depends on the coal type, combustion efficiency, and the pollution control equipment used. Essentially all of the mercury entering the furnace with the coal is vaporized and exists in the elemental form until the flue gases cool below ~600 °C (~1000 °F) [1][2][3]. The oxidation of mercury in coal-fired boiler systems is kinetically limited [1][2][3].…”

“…Essentially all of the mercury entering the furnace with the coal is vaporized and exists in the elemental form until the flue gases cool below ~600 °C (~1000 °F) [1][2][3]. The oxidation of mercury in coal-fired boiler systems is kinetically limited [1][2][3]. Because the concentration of mercury is very small in flue gas, any favorable mercury-oxidation reaction does not have the ability to promulgate itself.…”

“…The elemental sulfur then reacts directly with elemental mercury to form mercuric sulfide, and the ionic forms of sulfur react with ionic mercury, also forming mercuric sulfide. Equations (2)(3) give examples of these mercury stabilization reactions: S° + Hg° HgS (2) Na 2 S 4 + HgCl 2 HgS + 2NaCl + 3 S°…”